Recently, comprehensive researches have been performed for enhancing the quality of a semiconductor device having a silicon layer on a GaAs substrate. However, there are two major problems in applying such a semiconductor element to MMIC's (Monolithic Microwave Integrated Circuits) and digital IC's. One is that the silicon substrate bends in response to a residual thermal tensile stress arising in the substrate due to the difference in the thermal expansion coefficients of silicon and GaAs, thereby causing cracking in the GaAs layer. The other problem is that the electrical resistance of the silicon substrate is poor, thereby causing poor high frequency characteristics of the element.
As is disclosed by "K. Kasai, et al., in J. Appl. Phys. 60(1986) 1", a (111) face GaAs epitaxial layer is directly grown on a sapphire (0001) face sapphire has a thermal expansion coefficient close to that of GaAs and is a good insulator. However, the crystal orientation of (111) face of GaAs is not suitable for practical use.
Recently, T. P. Humphreys et al. in, Appl. Phys. Lett. 54(1989) 1687 reported using a Si-on-sapphire (SOS) substrate where (100) Si epitaxial layer is grown on a (1102) face sapphire substrate. However, the surface configuration of the semiconductor layer produced thereon is rough and resulting in poor device characteristics and that in the preciseness of fine pattern processing.
More recently, J. B. Posthill et al., in Appl. Phys. Lett. 55(1989) 1756 reported an improved surface configuration. However, epitaxial growth of GaAs on the SOS substrate is technically difficult since antiphase boundaries and high dislocation density remain at the surface of the semiconductor layer.
FIG. 4 is a diagram showing the surface of the GaAs layer which is produced on a spherical silicon substrate, disclosed in Japanese Journal of Applied Physics, 25 (1986) L789. In the figure, reference numeral 25 designates a spherical shaped silicon substrate and reference numeral 26 designates a GaAs layer. Reference numerals 27a to 27d designate portions having a mirrorlike smooth surface on the surface of the GaAs layer 26 and reference numeral 28 designates a portion which is hazy.
As shown in FIG. 4, it is usual to use the surfaces 27a, 27b, 27c, and 27d which are tilted toward &lt;110&gt;, &lt;110&gt;, &lt;110&gt;, and &lt;110&gt; direction, respectively, by several degrees from the (001) face, in order to grow GaAs on the silicon substrate. Then, the GaAs crystal which is grown on the surface is monocrystalline and produces a mirrorlike smooth surface. However, the surface which are offset toward the &lt;100&gt;, &lt;010&gt;, &lt;100&gt;, or &lt;010&gt; direction from the (001) face, respectively, are not monocrystalline and anti-phase boundaries remain to produce a hazy surface 28 as is well known, because silicon is an element while GaAs produced thereon is a compound.
It is well known that (100) silicon substrate grows on the R face (1102) sapphire substrate. There is an example using an SOS substrate comprising silicon on sapphire, being 1 to 8 degrees offset toward the &lt;110&gt;, &lt;110&gt;, &lt;110&gt;, or &lt;110&gt; direction (collectively called as [110] direction), from the (100) face while growing GaAs or an other III-V group compound semiconductor on the thus produced SOS substrate, as disclosed in Japanese Laid-open Patent Publication No. Hei. 1-173709. However, since the distribution of the mirrorlike smooth surface region of GaAs on the silicon substrate and on the SOS substrate are different even when the offset direction and offset angle of the SOS substrate are determined from the consideration of the mirrorlike smooth surface distribution of GaAs produced on the spherical silicon substrate as shown in FIG. 4, a problem remains in that GaAs surface produced thereon may or may not produce a mirrorlike surface. This may be attributed to the fact that sapphire has a 3-fold rotational symmetry, while Si and GaAs crystals have 4-fold rotational symmetry.